The present invention relates to fuel cells and, more particularly, to fuel cells arranged in a stack and held in compression.
Fuel cell stacks typically comprise a plurality of fuel cells stacked one upon the other and held in compression with respect to each other. The plurality of stacked fuel cells form a fuel cell assembly which is compressed to hold the plurality of fuel cells in a compressive relation. Typically, each fuel cell comprises an anode layer, a cathode layer, and an electrolyte interposed between the anode layer and the cathode layer. The fuel cell assembly requires a significant amount of compressive force to squeeze the fuel cells of the stack together. The need for the compressive force comes about from the internal gas pressure of the reactants within the fuel cells plus the need to maintain good electrical contact between the internal components of the cells. Generally, the per area unit force is about 195-205 psi total which is distributed evenly over the entire active area of the cell (typically 77-155 square inches for automotive size stacks). Thus, for a fuel cell with an area of about 80 square inches, the typical total compressive force of these size stacks is about 15,500 to 16,500 pounds.
Typical prior art fuel cell stack structures focused on the use of rigid end plates to apply and maintain a compressive force on the fuel cell assembly. The fuel cell assembly to be compressed is interposed between a pair of rigid end plates. The end plates are then compressed together and held in a spaced relation to maintain the compressive force. The end plates can be held in a spaced relation by a variety of means. For example, tie rods that extend through the end plates can be used to impart a compressive force on the end plates and hold the end plates in a spaced relation. The tie rods are typically external to the fuel cell assembly and are situated along the periphery of the end plates. Side plates that extend along a length of the fuel cell assembly and are attached to the end plates have also been used to hold the end plates in a spaced relation and maintain a compressive force on the fuel cell assembly.
Due to the high compressive force that must be imparted on the fuel cell assembly and the size of the active area of the fuel cell assembly over which the compressive force must be imparted, rigid end plates that are maintained in a spaced relation by means along the periphery of the rigid end plates have a tendency to deflect and not impart a generally uniform compressive force over the entire active area of the fuel cell assembly. That is, the central portion of the rigid end plates deflect and the force applied to the active area below the central portions of the rigid end plates is not as great as the force applied to the active area along the periphery of the rigid end plates.
Prior art attempts to provide a generally uniform compression distribution over the active area have included very thick rigid end plates with external tie rods, rigid end plates with internal tie rods passing through the fuel cell assembly, semi-rigid end plates with a cavity for a gas bladder, and the use of discreet force exerting members, such as screws, that are positioned above the central portions of the end plates and can be selectively moved relative to the end plates to impart a compressive force along the central portions of the end plates.
In the rigid end plate with external tie rods, threaded tie rods extend from the periphery of an upper end plate along the outside of the fuel cell assembly to the periphery of the lower end plate so the total compressive force is carried by the tie rods. The end plate must be thick enough so that a small (about less than 1 mil per cell) total deflection is achieved. The disadvantage of this system is that the end plate must be very thick as compared with all other options since the total end plate span is the largest and no other method is employed to generate an even force over the entire active area.
In the rigid end plate with internal tie rods through the fuel cell assembly, the tie rods extend through the center of the fuel cells to allow the placement of the tie rods nearer to the central portion of the end plates. Now the total span of the bending force is not extended over the entire width of the upper end plates but rather a shorter span is achieved. This arrangement has the advantage of reducing span length of the end plates resulting in the ability to use a thinner end plate but has the disadvantage that it requires complex bipolar plate sealing mechanisms to enable the tie rods to pass through the fuel cell assembly.
In the semi-rigid end plate with a cavity for a gas bladder, the lower face of the upper end plate is hollowed out, a bladder is positioned in the end plate cavity, and the bladder is pressurized to provide the desired compressive loading of the stack. The upper end plate itself is now allowed to bend somewhat while the bladder maintains uniform force distribution over the entire active area. This arrangement has the advantage that the structural component of the upper end plate can be made thinner since it is allowed to flex considerably but has the disadvantage that it requires a cavity in the end plate with the result that the overall thickness of the end plate is significantly increased.
In the use of discreet force exerting members, the discreet force exerting members are positioned above the central portions of the end plates and are selectively moved relative to the end plates to impart a compressive force along the central portions of the end plates. This arrangement has the advantage of fine tuning the compressive force applied to the various locations of the end plates above which a discreet force exerting member is positioned but has the disadvantage that it requires extra mechanisms to maintain the discreet force exerting members and requires an iterative process of tightening the various discreet force exerting members to attain a generally uniform force distribution on the active area of the fuel cell assembly.
Therefore, what is needed is a fuel cell stack structure that has end plates that apply a generally uniform compressive force along the active area of the fuel cell assembly without requiring excessively thick end plates or the use of extra means to apply a compressive force to the central portions of the end plates.
The present invention is directed to an apparatus for providing a fuel cells stack structure that compresses the fuel cell assembly and imparts a generally uniform compressive force over the active area of the fuel cell assembly. More specifically, this invention is directed to variations on the design of the end plates that improve the distribution of the compressive force over the active area of the fuel cell assembly.
An electro-chemical fuel cell stack of the present invention comprises a plurality of fuel cells arranged in a stacked configuration to form a fuel cell assembly. The fuel cell assembly has opposite first and second ends. First and second end plates with opposite inner and outer surfaces are disposed adjacent the respective first and second ends of the fuel cell assembly. The inner surfaces of the end plate face the ends of the fuel cell assembly. The first and second end plates are held in a spaced relation so that the first and second end plates impart a compressive force on the fuel cell assembly. The inner surface of at least one of the first or second end plates is contoured so that a generally uniform compressive force is imparted on the fuel cell assembly. The contoured inner surface can be contoured so that the inner surface extends from the at least one end plate toward the end of the fuel cell assembly. Preferably, the end plate with the contoured inner surface has contours so that the end plate has a thickness that increases from a periphery of the end plate to a center of the end plate with a maximum thickness being in the center of the end plate. Optionally, the inner surfaces of both the first and second end plates can be contoured to extend from the first and second end plates toward the respective first and second ends of the fuel cell assembly so that a generally uniform compressive load is imparted on the fuel cell assembly.
In an alternate embodiment of the present invention, an electro-chemical fuel cell stack comprises a plurality of fuel cells arranged in a stacked configuration to form a fuel cell assembly. The fuel cell assembly has opposite first and second ends. First and second spacer plates with opposite inner and outer surfaces are disposed adjacent the respective first and second ends of the fuel cell assembly. The inner surfaces of the spacer plates face the ends of the fuel cell assembly. First and second end plates with opposite inner and outer surfaces are disposed adjacent the respective first and second spacer plates with the spacer plates being between the end plates and the ends of the fuel cell assembly. The inner surfaces of the end plates face the outer surfaces of the spacer plates. The first and second end plates are held in a spaced relation so that the first and second end plates impart a compressive force on the spacer plates and on the fuel cell assembly. At least one of the surfaces of at least one of the spacer plates or end plates is contoured so that a generally uniform compressive force is imparted on the fuel cell assembly.
In a different alternate embodiment of the present invention, an electro-chemical fuel cell stack comprises a plurality of fuel cells arranged in a stacked configuration to form a fuel cell assembly. The fuel cell assembly has opposite first and second ends. First and second terminal plates are disposed adjacent the respective first and second ends of the fuel cell assembly. First and second end plates are disposed adjacent the respective first and second terminal plates with the terminal plates being between the end plates and the ends of the fuel cell assembly. At least one terminal plate of the first and second terminal plates is attached to one of the first or second end plates so that stiffness of the at least one terminal plate contributes to stiffness of the attached end plate. The first and second end plates are held in a spaced relation so that the first and second end plates impart a compressive force on the fuel cell assembly. Optionally, the fuel cell stack can also comprise at least one spacer plate. The at least one spacer plate is interposed between the at least one terminal plate attached to the one of the first or second end plates. The at least one spacer plate is attached to the at least one terminal plate and to one of the first or second end plates so that stiffness of the at least one spacer plate contributes to the stiffness of the attached end plate.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.